High energy capacitive discharge ignition systems are used to ignite either a main fuel or a pilot fuel in a variety of devices including internal combustion engines and industrial burners. Conventional capacitive discharge ignition systems include an exciter circuit to excite or energize an igniter plug. When the igniter plug is energized, it generates a spark to ignite the fuel. An exciter circuit includes an energy storage capacitor, a charging circuit to charge the capacitor, and a discharge circuit through which the energy storage capacitor is discharged into the igniter plug.
The discharge circuit includes a switching device connected in series between the storage capacitor and the igniter plug. Typically, such capacitive discharge ignition systems have used spark gap tubes with two electrodes as the discharge circuit switching device to isolate the energy storage capacitor from the igniter plug while the capacitor is being charged. When voltage on the capacitor reaches the spark gap break-over voltage, the capacitor discharges across the two electrodes of the spark gap tube and energizes the igniter plug where a spark is produced.
Repeated discharges through the spark gap tube cause electrode erosion and other detrimental changes within the tube having the effect of lowering the tube's break-over voltage. As a result, the voltage level attained in the storage capacitor prior to discharge and the energy transferred from the storage capacitor to the igniter plug at the time of discharge decline over the lifetime of the spark gap tube.
Recently, there have been proposals to replace the spark gap tube with a solid state switch. Unlike the spark gap tube, a solid state switch would not exhibit the degradation over repeated discharges of the energy storage capacitor. The solid state switch, typically a thyristor, has two main terminals, one being an anode the other being a cathode and a gate terminal. In its blocking or off state, the thyristor does not conduct current between its anode and cathode terminals thereby blocking flow of current from the energy storage capacitor to the igniter plug. In its conducting or on state, the thyristor conducts discharge current between the anode and cathode terminals thereby providing a current path between the energy storage capacitor and the igniter plug.
Conventionally, the anode and cathode terminals are used to directly replace the electrodes of the spark gap tube, the thyristor being connected in series between the storage capacitor and the igniter plug. In the blocking state, the thyristor sustains capacitor voltage and blocks current flow from the capacitor to the igniter plug while the capacitor is being charged. Once the capacitor is fully charged, the thyristor conduction or on state is triggered in response to a control signal applied to the gate terminal thereby initiating conduction between the cathode and anode terminals. Discharge current from the storage capacitor flows across the thyristor and into the igniter plug. The signal to the gate terminal is made responsive either conditionally or unconditionally to a circuit that senses full voltage across the storage capacitor.
A valuable advantage of using a thyristor in a discharge circuit of a discharge ignition system is that it has a demonstrated operating life over 25 times that of a spark gap tube. Additionally, storage capacitor voltage and energy at the time of discharge remain essentially constant over the life of the thyristor.
However, there are significant disadvantages to replacing a spark gap tube with a conventional thyristor switch. These disadvantages stem from the high peak power requirements at the igniter plug necessary to ignite fuel under adverse conditions. In a thin film semiconductor type igniter plug, that is an igniter plug with semiconductor material between the igniter plug electrodes, the required trigger voltage to generate a spark is on the order of 5000 volts (V). Within less than one microsecond after spark inception, voltage across the igniter plug electrodes decays to between 50 and 100 V and remains at this voltage level for an additional few microseconds (called the dwell period) before stabilizing at about 25 V.
When discharge current through the discharge circuit rises rapidly at a rate di/dt of between 500 and 1000 amps per microsecond (A/.mu.s) during the dwell period, high peak power is generated by the coincidence of high igniter plug voltage (50-100 V) and peak current. If the rise of discharge current is delayed or the rate of rise is too low, the igniter plug electrode voltage will decay to the lower 25 V voltage level, before the peak current is attained and, therefore, peak power supplied to the igniter plug will be lower. Furthermore, inductive reactance in the discharge circuit, which opposes the rapid rise in current, requires that source voltage across the energy storage capacitor be above 2000 V. Accordingly, a spark gap tube in a typical discharge circuit sustains a source voltage of between 2000 V and 3000 V while the storage capacitor is charged and conducts discharge current that rises at a rate between 500 A/.mu.S and 1000 A/.mu.S to a peak of 2000 A.
Use of single thyristor, which can operate under the above conditions, to replace a spark gap tube in a discharge circuit has not been practical because such thyristors are large, expensive, and not readily available. In contrast, thyristors of moderate size and price which are generally available as off-the-shelf items are typically limited to an operating voltage of 1200 V, a pulse current peak of 1000 A and a current transition rate (i.e. di/dt) of 200 A/.mu.S. Use of a single off-the-shelf thyristor with the above limitations results in reduced peak power and spark energy for the igniter plug.
Various designs to overcome these limitations have been proposed but each of these designs suffer from a number of disadvantages.
1. Use of a plurality of thyristors connected in series. Connecting two or three of these thyristors in a series string provides the 2000 V to 3000 V standoff capability necessary for high peak power but it also adds to circuit cost and complexity. In addition to the extra devices, additional components (typically a diode, resistor and capacitor) must be connected across each thyristor to insure that voltage across the series string is shared equally by each thyristor when they are blocking capacitor voltage and when they are turning on. Furthermore, the gate circuit for a single device expands to include additional devices.
2. Use of a saturable core inductor (sometimes referred to as a delay reactor). To allow conventional thyristors to conduct current with the high transition rate necessary for high peak power, a saturable reactor may be connected in series with the thyristor. Such an arrangement, however, does not always produce the expected beneficial results. One type of thyristor that is typically used in a discharge circuit is a silicon controlled rectifier (SCR) which is a unidirectional thyristor, conducting current in only one direction. The General Electric SCR Manual, 5th edition, section 5.5.1, p. 141-142 (copyright 1972) teaches that a high rate of change of di/dt of on-state current while an SCR is in the process of turning on is capable of destroying the SCR.
During the turn on process of an SCR, only a small portion of the silicon die area around the gate electrode attachment conducts current due to a finite spreading velocity. If a fast rising current is permitted at turn on, a high current density occurs in a small conducting area of the die resulting in high switching losses. These high losses create excessive heating and are of a destructive nature to the thyristor device. To allow proper current spreading over the entire silicon die area before fast rise current is permitted, a saturable core inductor, referred to as a delay reactor, must be placed in series with the thyristor switch. Initially, when the thyristor turns on, the inductance of the reactor is high. This limits the rate of rise of current (di/dt) to less than a destructive value, typically 200 A/.mu.S, during the delay period that conduction current spreads across the die area.
Once the thyristor is in full conduction and current has risen to the level that causes magnetic saturation of the delay reactor's core material, the inductance becomes a small value. Current then rises safely at a rapid rate between 500 A/.mu.S and 1000 A/.mu.S. However, the use of a delay reactor to increase di/dt of the discharge current through a thyristor will not produce a high peak power if the igniter plug voltage is allowed to stabilize at a low level during the decay period. Stabilization of igniter voltage can be prevented by keeping igniter current below a few amps during the delay period while delay reactance is high. However, this is not practical since reliable spark initiation for worn or fouled igniter plugs requires at least 25 A spark current between the igniter plug electrodes.
Additionally, a delay reactor in the discharge circuit presents an obstacle to developing a sufficient trigger voltage at the exciter circuit output. Whereas voltage on a storage capacitor between 2000 V and 3000 V is sufficient to initiate and sustain a spark for one type of igniter plug (bulk or pellet semiconductor igniter plug), another type (thin film semiconductor igniter plug) requires 5000 V to reliably initiate the spark, and yet another igniter plug (surface gap igniter plug (sometimes incorrectly referred to as an air gap igniter plug)) requires 15000 V to 25000 V. Traditionally, a trigger circuit is connected between the discharge circuit switch and the exciter circuit output to generate the igniter plug's required spark inception voltage or trigger voltage in cases where it is higher than the capacitor's storage voltage. The trigger circuit generates a short duration (0.1 .mu.S to 1 .mu.S) high voltage pulse at the output of the exciter circuit that initiates the spark at the igniter plug.
After the spark is initiated and the high voltage pulse has passed, the spark is sustained by the lower voltage of the storage capacitor. A typical trigger circuit requires an input voltage with a fast rise time from a low impedance source. The high impedance output of the delay reactor is not compatible with the low impedance input required by the trigger circuit.
3. Use of a plurality of thyristors connected in parallel. The high current peak, typically 2000 A, conducted by the spark gap tube to produce high peak power at the igniter plug could be conducted by two or more thyristors with the above limitations when connected in parallel. But this approach suffers from the same disadvantage as using multiple devices to increase standoff voltage (i.e. multiple devices with their required ancillary circuits increase circuit cost and complexity).
Another problem not related to spark power or energy occurs after conduction of the discharge current when the thyristor must be allowed to recover its blocking state. If the thyristor does not recover its blocking state, it will conduct current from the charging power supply away from the storage capacitor and prevent recharge of the capacitor. The spark gap tube and thyristor are both regenerative devices and, as such, will switch out of conduction once their conduction current falls below a sustaining level that is characteristic of each device. The charging circuit charging current, which is typically less than one ampere, is below the sustaining current for a spark gap tube. Consequently, the spark gap tube turns off unaided after each discharge, allowing the storage capacitor to recharge. However, the thyristor has a much lower level of sustaining current (typically only 10 mA). Thus, a thyristor will be held in conduction by the charging circuit charging current that is typically above the thyristor sustaining current level unless other means are provided to momentarily reduce its conduction current to below the sustaining level.
One means of turning off a thyristor consists of momentarily turning off the charging circuit power supply until the thyristor has time to recover its blocking state. This method is practical for a power supply of the electronic high frequency switching variety wherein an existing electronics power switch can be cycled off and back on after each discharge with the addition of little or no extra control circuitry. It is more expensive to apply this method to a conventional charging power supply, which uses a line frequency, high voltage transformer to supply the charging current. In this case a power electronic switch with additional control circuitry must be added to cycle the transformer off and back on after each discharge.
Other methods of turning the thyristor off after each discharge that do not require the interruption of charging supply current work by diverting current away from the thyristor momentarily until the thyristor has time to recover its blocking state. One of these methods relies on resonant elements in the discharge circuit to reverse current momentarily in the thyristor at the end of each discharge pulse allowing the thyristor to turn off. These resonant circuit elements are inherently a part of some discharge circuits while in other discharge circuits such resonant circuit elements must be added increasing the cost of the circuit and resulting in loss of circuit efficiency.
Accordingly, there is a need for an exciter circuit that provides the igniter plug with both a rate of rise of current and a peak current that is substantially higher that the rise of current and peak current through the discharge circuit switch.
What is also needed is an exciter circuit that permits an increased storage capacitor voltage substantially above the voltage across the discharge circuit switch so that power supplied to the igniter plug is maximized and allows replacement of the spark gap tube with a single solid state discharge circuit switch.
What is also needed is an exciter circuit discharge circuit that provides the igniter plug with a fast rising current, which does not initially rise as a significantly lower rate thus allowing the igniter plug to attain high peak power before the igniter plug electrode voltage has had time to decay to a lower steady state voltage.
What is also needed is an exciter circuit trigger circuit that provides the igniter plug with an initial high voltage pulse to initiate the igniter plug spark at a voltage substantially above the source voltage across the storage capacitor.
What is also needed is an exciter circuit discharge circuit that inherently provides reverse current to the thyristor after each discharge pulse so that the thyristor has time to turn off without interrupting the flow of current from the charging circuit power supply or having to add additional circuit elements for turning the thyristor off.